The invention is directed to improvements in fuel injection pumps having a reciprocating pump piston.
In known fuel injection pumps of this type, the relief conduit of the pump work chamber is opened up by the regulating slide, in order to terminate the injection, and it remains open until the end of the compression stroke. Depending upon the axial position of the regulating slide, this opening of the relief conduit occurs after a variable compression stroke has been executed, or after the injection of an injection quantity corresponding to such a compression stroke. Particularly in fuel injection pumps operating at a high injection pressure, as is the case for direct injection engines, for instance, the high relieving speed of the fuel, which flows out counter to a very low fuel pressure, causes hollow spaces to appear in the pump work chamber and in the conduits communicating with it, possibly extending even as far as the pressure lines, depending upon the type of pressure valve involved. Because of the time available until the next compression stroke and the relatively low supply pressure while the pump work chamber is beinq refilled, these hollow spaces are often only inadequately refilled. This causes cavitation in the pump work chamber, on the one hand, and on the other hand results in a reduced actual injectible fuel quantity as compared with the set-point quantity set by the regulating slide. There is also a shift in the onset of fuel supply or fuel injection, because the hollow spaces must first be refilled with fuel, before the required injection pressure can be built up.
Because of this negative pressure in the pump work chamber, it is also possible for such a high pressure difference to develop at the opening valve of the fuel injection nozzle that gases can travel from the engine combustion chamber through the nozzle into the injection lines, before the nozzle needle has closed; this can cause soiling of the valve seat and attendant leakage, and it also amplifies the decrease in the setpoint injection quantity and the fluctuations in the injection onset mentioned above.